Hackers have accessed confidential details of an estimated 5,000 customers of an American banking group.
San Francisco-based Wells Fargo is notifying affected customers of the breach.
Details of the breach are contained in a letter that the bank sent to New Hampshire's attorney general. It appears to be one of several letters sent to different states where people's data has been compromised.
Written by the bank's senior company counsel, Peter McCorkell, it said, "This letter is to advise you of an information compromise incident which affects residents of New Hampshire.
"Wells Fargo Bank, N.A. has been advised by a reseller of consumer data, including consumer credit bureau data, of suspicious transactions made using Wells Fargo access codes. Our investigation confirmed that a significant number of unauthorized transactions had been made using Wells Fargo's codes. At this time, we do not know how our codes were compromised. We have notified the United States Secret Service, and it is investigating this matter."
Personal information including names, addresses, dates of birth, social security numbers, driver's licence numbers and, in some cases, credit account information was accessed by "unauthorized person or persons".
The bank initially estimated that hackers accessed details of 7,000 customers, although its latest estimate has decreased that number to 5,000.
The letter continued: "About 7,000 individuals are affected by this incident. At this time, we have apparently mailable addresses for only 2,410 of which nine are residents of New Hampshire. We are attempting to find usable addresses for the remaining affected individuals and will supplement the count for your state when we do."
In a letter to affected customers, the bank said, "We are writing to inform you that a third-party data provider has notified us that a Wells Fargo access code was used to gain unauthorized access to your personal information."
It continues: "We are working with the credit bureaus to ensure that any unauthorized inquiries under Wells Fargo's name will not affect your credit rating."
The bank is offering affected customers a free one-year membership of a service to monitor their credit files at the national credit reporting agencies for suspicious activities that could indicate identity theft.
It suggests that customers, as a precautionary measure, should contact all of their other banks, credit card companies, and providers of financial services to ask what further steps they advise.
A spokeswoman for the bank said, "Wells Fargo is taking this incident very seriously and we are taking significant steps to help consumers protect their financial accounts. This is an isolated incident, and we are actively supporting law enforcement in the investigation to identify those responsible."
Wells Fargo was hit by a data security breach two years ago when a computer containing confidential details about mortgage customers went missing.